TABLE OF CONTENTS We accomplished a great deal in Chapter 7 we took a product idea to a working prototype. But there is a little more design work to do before our prototype can be called a USB I/O device. Additionally, if we want to move our prototype out of the lab and into the marketplace, there are several more steps that we have to take. This chapter covers the requirements of building a USB compliant I/O device, and the assistance that the USB Implementers Forum (USB IF) will provide to ensure that your product is salable throughout the world.
The additional design steps involve the power handling of our I/O device, and the marketing of our product involves a Vendor ID. The USB IF also supplies a suite of tests to enable thorough testing of your product.
Let s recap the USB Specification s power requirements: There are two distinct operating modes of an I/O device, active and suspended. When active, our I/O device can draw up to 100 mA from Vbus and be classified as a low-power device, or it can draw up to 500 mA from Vbus and be classified as a high-power device. My recommendation is to strive to get under the 100 mA limit since your device will then be operational even on an unpowered hub. If your I/O device requires more than 500 mA, then it must be self-powered and will therefore need its own power cord.
There are also limits on the amount of current...
